To attain these performance levels, it is known practice to use, on the sources for the antennas, quad-arm exciters based on an orthogonal-mode junction coupler (also known by the abbreviation OMJ for “OrthoMode Junction”) comprising four coupling accesses and systems for recombining the polarizations. The function of the orthogonal-mode junction coupler is to extract or excite the two modes of linear polarization.
However, this device complicates the system for recombining the polarizations notably in respect of the routing of the guides with a set-up on two layers in order to perform this function. This complex recombination system therefore penalizes the size and mass of the sources. Moreover, the use of such an architecture on Gregorian antennas is more difficult to organize owing to the size of the source and the poor fields of view that are generated, affecting the radiation patterns.
As an illustration, FIG. 1 shows an exemplary embodiment of such an architecture in a dual-band configuration. The device comprises an orthogonal-mode junction coupler 10, one end of which is connected to a horn 12 by means of a transformation device. A second end is connected to a polarization separator 14 (also known by the abbreviation OMT for “OrthoMode Transducer”) by means of a cut-off filter 13. Each of the four coupling accesses of the coupler 10 is connected to a filtering arm 15. The outputs of these filtering arms 15 are recombined two by two by means of an “H”-divider 17, which is also called a “magic T”, with a load 19. The last access of each summer 17 corresponds to an input/output port of the device. Equally, the two accesses of the polarization separator 14 that are not connected to the cut-off filter 13 correspond to two other input/output ports of the device.
FIG. 2 shows a second type of architecture known from the prior art allowing the required performance levels to be obtained. This device comprises a horn 12 connected to a polarization separator 14 so as to separate the two modes of polarization of the signal and each of the two arms of said polarization separator 14 is then connected to a duplexer 16 so as to extract the two frequency bands that are present in the signal.
This second architecture has the advantage of having a smaller number of microwave components in order to perform the function of separating the frequency bands and the polarizations. However, it can be used only when frequency bands are sufficiently close together. Moreover, the use of an asymmetric polarization separator 14 makes separation of the polarizations more sensitive owing to the possible excitation of higher modes.
It is also known practice to use an orthogonal-mode junction coupler 10 having two coupling accesses. FIG. 3 illustrates an exemplary embodiment thereof. In this figure, one end of the orthogonal-mode junction coupler 10 is connected to a horn 12 by means of a polarization transformation device 11 and a second end is connected to a polarization separator 14 by means of a cut-off filter 13. Each coupling access of the coupler 10 is connected to a filtering arm 15. The two outputs of the polarization separator and the outputs of the filtering arms 15 define input/output ports of the device.
This architecture has the advantage of being simple and space-saving but affords a relatively low level of decoupling between the modes of polarization. This configuration affords a level of horizontal/vertical polarization decoupling of only approximately −18˜−22 dB, whereas the needs are −50 dB for assignments with fully developed monobeam coverage and −35 dB for multiple beams. This poor decoupling can be explained by the imbalance in the electrical field linked to the use of a single polarization coupling slot on the orthogonal-mode junction coupler.